1. Field of the Invention
The present invention relates generally to an oil recovery method and an oil recovery apparatus and, more particularly, to a method and apparatus for efficiently recovering oil spilled over the water surface of, for example, sea water by accidents or the like, by separating the oil from the water without using expensive and special recovery ships or recovery apparatuses of conventional types.
2. Background Information
Oil spilled over the ocean by oil field accidents, tanker accidents or the like has been recovered by special or multi-purpose recovery apparatuses for a suction-type oil recovery ship or a dredger-type oil recovery ship. However, since all of these apparatuses are expensive, the number of ships disposed at various areas, including harbors, is limited. Furthermore, large-sized facilities are required for mooring or storing these apparatuses. Additionally, if oil spill accidents or the like occur at areas far from the harbors in which these apparatuses are disposed, it becomes difficult to take immediate actions to conduct a recovery plan.
There is also known an excavating bucket system by which oil is recovered by skimming it using excavating buckets of floating cranes or gravel gathering and carrying ships (i.e., gut ships).
At the time of the heavy oil spill accident involving the oil tanker Nakhodka in the Japanese Sea in January 1997, various oil recovery machines were used. At that time, since the spilled oil was heavy oil having a high viscosity, suction-type oil recovery apparatuses were poor in efficiency, and the most efficient method of removing the oil was by skimming it with buckets of the gut ships. However, when using the excavating bucket system, a large amount of sea water and the like is recovered together with the spilled oil in a mixed state. Accordingly, since the recovery efficiency of the oil is low and a large amount of water is recovered together with the oil in a mixed state, the transport volume of the recovered oil is substantial, and the facilities (pits) on the land for receiving the recovered oil for post-treatment are required to be large in capacity.
Under such circumstances, as a method for more efficiently recovering oil as compared with the conventional methods, the present inventors have invented a bucket of a grab type having a slanting bottom plate and a shell provided with apertures or slits. The present inventors have filed Japanese Patent Application No. 130156 in 1999 covering the grab-type bucket together with a measurement technique for measuring oil viscosity and a method for oil recovery. However, from tests and observations made after such filing, it was found that before the oil and water are completely separated, a part of the oil tends to drop through the apertures or slits provided on the upper portion of the bucket. There is therefore a demand for a technique by which the water and oil are further securely separated.
Further, in a recovery operation using the foregoing grab-type bucket, in the step of closing the bucket, the water surface in the bucket is raised and the floating oil on the surface and the water may sometimes flow out of the bucket. Thus, there is also demand to further increase the recovery efficiency of the oil.
Moreover, with respect to heavy oil having a high viscosity, the foregoing grab-type bucket system can separate the oil and water efficiently depending upon the bucket. However, since tankers navigating the waters near the Sakhalin sea and the Okhotsk sea mostly carry crude oil, and the viscosity of crude oil is lower than that of heavy oil, a phenomenon has been seen that it is impossible to efficiently separate the oil and water. It was found that this phenomenon is attributable to the fact that in the case of crude oil, the time-lag between the drainage of water from the bucket and the start of dropping of the oil is short, while in the case of an oil having a low viscosity, the oil drops immediately after the dropping of water, whereby separation of the oil and water becomes difficult.
Further, when the spilled oil surging to the seashore is treated with heavy machines, the treated oil mostly forms a mixture with sand or gravel, and then forms a large amount of secondary contaminants, whereby secondary separation operations by manpower are required. Furthermore, in the secondary separation operations, since the separation of oil components adhered to the sand or the like is extremely difficult, it has been reported that the secondary contaminants are left as they are for a long period of time. Further, there have been reported serious examples of secondary pollution, for example, effusion of oil incorporated in sediments, and therefore there are few techniques for efficiently recovering a large amount of oil at the waterside.
The present invention has been made in view of the foregoing drawbacks in the conventional art.
It is an object of the present invention to provide an oil recovery method and an oil recovery apparatus by which spilled oil and water are efficiently separated at the initial step of the recovery operation to increase the recovery efficiency of the spilled oil.
It is another object of the present invention to provide a recovery apparatus of a bucket system type by which the separation of water and the spilled oil to be recovered can easily be confirmed and the recovery efficiency of oil only is high.
Still another object of the present invention is to provide an oil recovery apparatus which can be manufactured at low cost and can be easily transported to areas affected by oil spillage.
Another object of the present invention is to provide an oil recovery apparatus which can be housed in facilities that are simple and small-sized, since the apparatus is adapted for use only in emergencies such as oil spill accidents.
Still another object of the present invention is to provide an oil recovery method by which oil recovery is conducted efficiently while preventing water and sand from penetrating even if the recovery is made on the seashore or the like.
The foregoing and other objects of the present invention are carried out by an oil recovery method comprising the steps of skimming water and oil having a preselected viscosity and floating on the water surface using a bucket, separating the water and oil to a lower portion and an upper portion of the bucket, respectively, utilizing a difference in specific gravities thereof, draining the water located at the lower portion of the bucket through apertures disposed on a bottom plate of the bucket and having a preselected aperture diameter and length so that the oil located at the upper portion of the bucket gradually flows into the apertures at a rate in accordance with the oil viscosity after the water is drained, and discharging the oil from the bucket before the oil drains out of the apertures.
According to the present invention, the bottom plate of the bucket is mounted for undergoing movement between a closed position and an open position, and the discharging step comprises discharging the oil from the bucket by opening the bottom plate. In an alternative embodiment, the discharging step comprises discharging the oil from the bucket by inclining the bucket.
Thus the present invention relates to an improvement of a bucket for skimming the spilled oil, by which the oil and water can efficiently be separated and the recovery rate can be increased. This is accomplished by providing the apertures on the bottom plate of the bucket for the purpose of removing or taking out water from the skimmed mixture of oil and water to recover the oil only.
The present inventors have conducted various experiments with the apertures of on the bottom plate of the bucket for separating water from the mixture of oil and water. As a result, it was found that Hagen-Poiseuille""s law is applicable as an approximate formula. In Hagen-Poiseuille""s law, the flow rate per unit time can be determined by the viscosity of a liquid, the diameter of a tube, the length of the tube, and the pressure difference between both ends of the tube, and this relation is represented by the following formula:
((xcfx80a4)/8xcex7)xc3x97((P1xe2x88x92P2)/1) 
where xe2x80x9caxe2x80x9d is the radius of a tube, xe2x80x9clxe2x80x9d is the length of the tube, xe2x80x9cP1xe2x88x92P2xe2x80x9d is the pressure difference between both ends of the tube, and xe2x80x9cxcex7xe2x80x9d is the viscosity coefficient of a liquid.
The viscosity of oil is far higher than that of water. When the mixture of oil and water is skimmed by the bucket, they separate from each other so that water is located at the lower portion of the bucket and oil is located at the upper portion of the bucket by the difference in the specific gravities of oil and water. Then, the water located at the lower portion of the bucket rapidly drains away through the apertures. After the drainage of water, the oil which has a higher viscosity gradually moves downward into the apertures and does not drain away immediately after the drainage of water. Namely, before the oil starts to drain away from the apertures, there is a short period of time wherein the dropping of oil from the apertures stops. The present inventors intend to utilize the short period of time wherein the dropping stops to discharge the oil remaining in the bucket completely.
The short period of time wherein the dropping stops can be optionally set with the approximate formula of Hagen-Poiseuille""s law by choosing the diameter and length of the apertures depending upon the viscosity of the oil to be recovered. The short period of time wherein the dropping of oil from the apertures does not yet start should be used for the operation time necessary to transport the bucket to a recovery tank or the like, in order to discharge the oil remaining in the bucket completely. This time is usually adjusted to be from about 10 to 40 seconds, preferably from about 20 to 30 seconds, taking the operation efficiency and the like into consideration.
It was confirmed by testing that the viscosity of crude oil is about 5xc3x97103 cSt (mm2/s) after 3 days passed in a no-wind laboratory test condition. Furthermore, as a result of a test under an air-blowing condition in a laboratory, a state where the viscosity became at least about 5xc3x97103 cSt (mm2/s) within 48 hours was confirmed. In nature, since the vaporization of volatile components is accelerated by wind and wave conditions, it is believed that the viscosity will mostly reach the foregoing numerical value after about 48 hours. Furthermore, by the emulsification due to waves, droplets and bubbles are mixed in the oil layer which achieves a mousse-like state, and the viscosity of the oil is thereby further increased, whereby the viscosity may often become at least about 104 cSt (mm2/s) within 48 hours.
It is considered that the recovery of spilled crude oil is accompanied by the dangers of ignition or injury to the health of individuals due to breathing of injurious gases or the like contained in the volatile components until the majority of the volatile components vaporize from the crude oil. Thus it is concluded that it is dangerous to start the recovery operation before the viscosity becomes at least about 5xc3x97103 cSt (mm2/s).
Thus, in the state where the crude oil is not yet emulsified after the volatile components are vaporized from the crude oil, the viscosity of crude oil is at most 104 cSt (mm2/s) Accordingly, as the operable range when the crude oil that is not yet emulsified is recovered, it is preferred to set the level of oil viscosity to 103 cSt (mm2/s). Furthermore, as the operable range when the crude oil that is already emulsified is recovered, it is preferred to set the level of viscosity to 104 cSt (mm2/s).
If the aperture size of the bucket is too large, water will quickly drain away from the bucket and water and the dropping of oil will start before the bucket is transported to the recovery tank or the like. Accordingly, depending upon the operation time, it is preferred to adjust the aperture diameter to be from about 1 to 10 mm, more preferably from about 3 to 7 mm. Further, within usual operation time, it is preferred to adjust the aperture diameter to about 5 mm, by which it is possible to handle oil having a higher viscosity.
In the separation of water and oil, it is preferred to provide apertures each having a generally cylindrical shape and a length of from about 8 to 40 times the diameter of the aperture, depending upon the viscosity of the oil, to secure the time before the oil starts to drop after the completion of drainage of water from the apertures of the bucket. In usual cases, when the viscosity of oil is at a level of about 103 cSt (mm2/s), if the aperture diameter is set to be 3 mm, it is preferred to set the length of the apertures to be about 100 mm. Further, when the viscosity of oil is at a level of at least 104 cSt (mm2/s), if the aperture diameter is set to be 5 mm, it is preferred to set the length of the apertures to be about 43 mm.